Water-based ink can reduce the risk of fire and the toxicity such as mutagenicity, which are problems of oil-based ink. Therefore, water-based ink has been widely used in inkjet recording applications other than industrial applications.
A water-based inkjet recording ink that uses a pigment has an advantage of providing excellent light resistance. However, if the pigment is not sufficiently pulverized, droplets cannot be stably discharged and the gloss of images to be formed is not improved. It is also difficult to stably hold pulverized pigment particles, which poses a large problem such as clogging of nozzles caused by aggregation and precipitation of the pigment. The precipitation due to the aggregation of the pigment poses a serious problem in that the yield and quality are decreased in the production process of a water-based pigment dispersion liquid, which is an intermediate of ink.
To address such problems, a method in which a pigment is dispersed in a water-based medium using a polymeric dispersant has been investigated. However, in general, it is difficult to disperse a pigment using only a polymeric dispersant so that pigment particles have a small particle size and to stably hold the dispersion of the pigment for a long time. In particular, there are not many yellow pigments having excellent light resistance. Thus, it is difficult to produce a water-based pigment dispersion liquid and a water-based inkjet recording ink that have excellent light resistance, dispersibility, and dispersion stability by using limited types of pigments.
A water-based pigment dispersion liquid containing Pigment Yellow 155, which is a bis-acetoacetallylide pigment, dispersed using a styrene-acrylic acid copolymer as a dispersant is known. See Japanese Unexamined Patent Application Publication No. 2005-060419.
The water-based pigment dispersion liquid produced in Japanese Unexamined Patent Application No. 2005-060419 has excellent color development and light resistance, which are strong points of Pigment Yellow 155. However, when the water-based pigment dispersion liquid is used for an inkjet ink that needs to have good jettability and low viscosity, the pigment is not always sufficiently dispersed and the precipitation caused by aggregation of the pigment during storage cannot be sufficiently suppressed. In addition, to meet a recent demand for high-gloss image quality equivalent to quality of film photos, pigment particles need to be dispersed so as to have a smaller particle size. Therefore, there has been a strong demand for addressing the above-described problems.
The precipitation caused by aggregation of a pigment has been significantly reduced and the dispersion stability of the water-based pigment dispersion liquid has been significantly improved by adding a diol compound having a specific structure in a kneading step of a pigment to prepare a pigment dispersion which contains the diol compound and is in a solid form at room temperature and then by dispersing the pigment dispersion in a water-based medium. See Japanese Unexamined Patent Application Publication No. 2007-254735 However, since Pigment Yellow 155 is a pigment whose particles are not easily dispersed finely and pigments of different colors used together in an inkjet recording method provide higher gloss, there has been a demand for further improving the gloss of formed images by finely dispersing a pigment.